1. Field of the Invention
This invention relates to a surface plasmon sensor for quantitatively analyzing a material in a sample utilizing generation of surface plasmon, and more particularly to a surface plasmon sensor in which the light source is improved to improve accuracy in analysis.
2. Description of the Related Art
In metal, free electrons vibrate in a group to generate compression waves called plasma waves. The compression waves generated in a metal surface are quantized into surface plasmon.
There have been proposed various surface plasmon sensors for quantitatively analyzing a material in a sample utilizing a phenomenon that such surface plasmon is excited by light waves. Among those, one employing a system called xe2x80x9cKretschmann configurationxe2x80x9d is best known. See, for instance, Japanese Unexamined Patent Publication No. 6(1994)-167443.
The plasmon sensor using the Kretschmann configuration basically comprises a dielectric block shaped, for instance, like a prism, a metal film which is formed on one face of the dielectric block and is brought into contact with a sample, a light source emitting a light beam, an optical system which causes the light beam to enter the dielectric block so that the light beam is reflected in total reflection at the interface of the dielectric block and the metal film and various angles of incidence of the light beam to the interface of the dielectric block and the metal film including an angle of incidence at which surface plasmon is generated can be obtained, and a photodetector means which is able to detect the intensity of the light beam reflected in total reflection from the interface for the various angles of incidence.
In order to obtain various angles of incidence of the light beam to the interface, a relatively thin incident light beam may be caused to impinge upon the interface while deflecting the incident light beam or a relatively thick incident light beam may be caused to converge on the interface so that components of the incident light beam impinge upon the interface at various angles. In the former case, the light beam which is reflected from the interface at an angle which varies as the incident light beam is deflected may be detected by a photodetector which is moved in synchronization with deflection of the incident light beam or by an area sensor extending in the direction in which reflected light beam is moved as a result of deflection. In the latter case, an area sensor which extends in directions so that all the components of light reflected from the interface at various angles can be detected by the area sensor may be used.
In such a plasmon sensor, when a light beam impinges upon the metal film at a particular angle of incidence xcex8sp not smaller than the angle of total internal reflection, evanescent waves having an electric field distribution are generated in the sample in contact with the metal film and surface plasmon is excited in the interface between the metal film and the sample. When the wave vector of the evanescent waves is equal to the wave number of the surface plasmon and wave number matching is established, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light reflected in total reflection from the interface of the dielectric block and the metal film sharply drops.
When the wave number of the surface plasmon can be known from the angle of incidence xcex8sp at which the phenomenon of attenuation in total reflection takes place, the dielectric constant of the sample can be obtained. That is,       Ksp    ⁡          (              ω        ~            )        =                    ω        ~            c        ⁢                                                      ε              m                        ⁡                          (                              ω                ~                            )                                ⁢                      ε            s                                                              ε              m                        ⁡                          (                              ω                ~                            )                                +                      ε            s                              
wherein Ksp represents the wave number of the surface plasmon, xcfx89 represents the angular frequency of the surface plasmon, c represents the speed of light in a vacuum, and xcex5m and xcex5s respectively represent the dielectric constants of the metal and the sample.
When the dielectric constant xcex5s of the sample is known, the concentration of a specific material in the sample can be determined on the basis of a predetermined calibration curve or the like. Accordingly, a specific component in the sample can be quantitatively analyzed by detecting the angle of incidence xcex8sp at which the intensity of light reflected in total reflection from the interface of the prism and the metal film sharply drops.
In the conventional plasmon sensor of the type described above, there has been generally used a laser as the light source. Especially when a single mode laser is used, the curve of attenuation in total reflection becomes sharper and a high sensitive measurement can be realized. However even such a laser is used, an accuracy of measurement cannot be always high.
In view of the foregoing observations and description, the primary object of the present invention is to provide a surface plasmon sensor in which a sufficiently high accuracy of measurement can be realized.
The surface plasmon sensor of the present invention comprises a dielectric block, a metal film, a light source emitting a light beam, an optical system, and a photodetector means which are described above and is characterized in that a laser provided with an oscillation wavelength stabilizing means for stabilizing the wavelength at which the laser oscillates is used as the light source.
A semiconductor laser, which is advantageous in reducing the overall size of system, can be suitably used as the laser. In this case, the oscillation wavelength stabilizing means may comprise, for instance, a beam feedback optical system which feeds a part of a laser beam emitted from the semiconductor laser back to the semiconductor laser and a wavelength selector such as a grating or a band pass filter which selects the wavelength of the laser beam to be fed back to the semiconductor laser.
In the case where a bulk grating is used as the wavelength selector, the beam feedback optical system may comprise a beam splitter means which is disposed on the optical path of the laser beam traveling from the semiconductor laser to the dielectric block and splits a part of the laser beam and a reflective grating which reflects the laser beam split by the beam splitter means to retrace its path, and the reflective grating may double as the wavelength selector.
It is possible to form the beam feedback optical system and the wavelength selector by a partial reflection type grating which is disposed on the optical path of the laser beam traveling from the semiconductor laser to the dielectric block and reflects a part of the laser beam toward the semiconductor laser.
Further it is possible to form the beam feedback optical system and the wavelength selector by a reflective grating which reflects toward the semiconductor laser a rearward laser beam emitted from the semiconductor laser in the direction opposite to the laser beam traveling from the semiconductor laser to the dielectric block.
Further, the oscillation wavelength stabilizing means may comprise a combination of a beam feedback optical system comprising a beam splitter means which is disposed on the optical path of the laser beam traveling from the semiconductor laser to the dielectric block and splits a part of the laser beam and a mirror which reflects the laser beam split by the beam splitter means to retrace its path and a narrow-band pass filter disposed on the optical path of the laser beam between the mirror and the semiconductor laser.
Further, the oscillation wavelength stabilizing means may comprise a combination of a beam feedback optical system comprising a half-silvered mirror which is disposed on the optical path of the laser beam traveling from the semiconductor laser to the dielectric block and reflects a part of the laser beam toward the semiconductor laser and a narrow-band pass filter disposed on the optical path of the laser beam between the half-silvered mirror and the semiconductor laser.
Further, the oscillation wavelength stabilizing means may comprise a combination of a beam feedback optical system comprising a mirror which reflects toward the semiconductor laser a rearward laser beam emitted from the semiconductor laser in the direction opposite to the laser beam traveling from the semiconductor laser to the dielectric block and a narrow-band pass filter disposed on the optical path of the rearward laser beam between the mirror and the semiconductor laser.
As the wavelength selector, may be used a fiber grating comprising an optical fiber which has a plurality of refractive index varying portions formed in the core at regular intervals and reflects and diffracts a laser beam.
In the case where a fiber grating is used as the wavelength selector, the beam feedback optical system may comprise a beam splitter means which is disposed on the optical path of the laser beam traveling from the semiconductor laser to the dielectric block and splits a part of the laser beam and a fiber grating which reflects the laser beam split by the beam splitter means to retrace its path, and the fiber grating may double as the wavelength selector.
It is possible to form the beam feedback optical system and the wavelength selector by a partial reflection type fiber grating which is disposed on the optical path of the laser beam traveling from the semiconductor laser to the dielectric block and reflects a part of the laser beam toward the semiconductor laser.
Further it is possible to form the beam feedback optical system and the wavelength selector by a fiber grating which reflects toward the semiconductor laser a rearward laser beam emitted from the semiconductor laser in the direction opposite to the laser beam traveling from the semiconductor laser to the dielectric block.
It is possible to stabilize the oscillation wavelength of the laser without feeding back the laser beam. For example, the oscillation wavelength of the laser can be stabilized by use of a DFB (distributed feedback) laser or a DBR (distributed Bragg reflector) laser as the light source.
Further the oscillation wavelength stabilizing means need not be limited to those described above and, for instance, a means for electrically controlling the laser drive current and/or the temperature of the laser may be used as the oscillation wavelength stabilizing means.
We have found that the problem that it is difficult to obtain a high accuracy in measurement in the conventional plasmon sensor using a laser as the light source is due to fluctuation in the oscillation wavelength of the laser. That is, fluctuation in the oscillation wavelength of the laser affects the condition of generation of the surface plasmon, which generates noise in the surface plasmon detecting signal (a signal representing the intensity of light reflected in total reflection from the interface of the dielectric block and the metal film) and deteriorates the accuracy in measurement.
Accordingly, in the surface plasmon sensor of this embodiment, fluctuation in the oscillation wavelength of the laser can suppressed by the oscillation wavelength stabilizing means and generation of the aforesaid noise is suppressed, whereby the accuracy in measurement can be improved.